26/11/2012 – Observatoire de Paris - 1 - Analysis of wind speed evolution over ocean derived from altimeter missions and models M. Ablain (CLS)

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Presentation transcript:

26/11/2012 – Observatoire de Paris Analysis of wind speed evolution over ocean derived from altimeter missions and models M. Ablain (CLS)

26/11/2012 – Observatoire de Paris The power of the return signal is related to the wind-induced roughness of the sea-surface. Wind speed is then estimated from empirical formulae. Wind direction cannot be resolved. Wind measurement from altimetry

26/11/2012 – Observatoire de Paris Overview The Global Mean Wind Speed (GMWS) over ocean derived from altimeters has been calculated:  Using T/P, Jason-1 and Jason-2 data and from Sigma-0 and SWH parameters.  Linking missions together to obtain an homogeneous time series from 1993 to 2012 The GMWS evolution highlight s a significant rise : ~2 cm.s-1/yr over all the period with inter- annual variations ( m/s-1) from 1997 to 2012.

26/11/2012 – Observatoire de Paris Overview We have also computed the GMWS over ocean derived from models:  Using ERA-interim, NECP and ECMWF operational models.  Interpolating the models on altimeter ground tracks (TP, J1, J2) From ERA-interim : the drift is smaller around 0.6 cm.s-1/yr without significant inter-annual signal

26/11/2012 – Observatoire de Paris Overview We have also computed the GMWS over ocean derived from models:  Using ERA-interim, NECP and ECMWF operational models.  Interpolating the models on altimeter ground tracks (TP, J1, J2) From NCEP : the drift is about 0.8 cm.s-1/yr without significant inter-annual signal

26/11/2012 – Observatoire de Paris Are there error in the GMWS evolution derived from altimetry ?

26/11/2012 – Observatoire de Paris The originality of the approach is to perform cross-comparisons between wind speed estimations deduced from measurements and from ERA-interim reanalysis produced by ECMWF. Thanks to the very good temporal correlation between the global wind speed evolution issued from altimeters and the atmospheric reanalysis, the detection of small drifts or jumps is possible in the altimeter time series. Study recently published : Detection of Long-Term Instabilities on Altimeter Backscatter Coefficient Thanks to Wind Speed Data Comparisons from Altimeters and Models (ID: DOI: / ) Journal: Marine Geodesy Estimated Publication date - 27 Nov 2012 (Online), 30 Nov 2012 (Print) Authors: Michael Ablain, Sabine Philipps, Marine Urvoy, Ngan Tran & Nicolas Picot Detection of Long-Term Instabilities on Altimeter Backscatter Coefficient Thanks to Wind Speed Data Comparisons from Altimeters and Models

26/11/2012 – Observatoire de Paris Ocean Wind speed stability over Jason-1/Envisat period  NCEP is more consistent than ERA-interim with altimeter wind speed in terms of global annual signal : 6 cm.s-1 amplitude difference. We have calculated global ocean wind speed differences between altimeters (Jason- 1, Envisat) and models (NCEP,ERA-interim) After adjusting periodic signals, drifts are highlighted on Jason-1 and Envisat wind speed by cross-comparisons with models

26/11/2012 – Observatoire de Paris Ocean Wind speed stability over Jason-1/Envisat period  NCEP is more consistent than ERA-interim with altimeter wind speed in terms of global annual signal : 6 cm.s-1 amplitude difference.  Jason-1 : ~10 cm.s-1 drift detected on 2004 (not seen on models and Envisat) We have calculated global ocean wind speed differences between altimeters (Jason- 1, Envisat) and models (NCEP,ERA-interim) After adjusting periodic signals, drifts are highlighted on Jason-1 and Envisat wind speed by cross-comparisons with models

26/11/2012 – Observatoire de Paris Ocean Wind speed stability over Jason-1/Envisat period  NCEP is more consistent than ERA-interim with altimeter wind speed in terms of global annual signal : 6 cm.s-1 amplitude difference.  Jason-1 : +10 cm.s-1 drift detected on 2004 (not seen on models and Envisat) We have calculated global ocean wind speed differences between altimeters (Jason- 1, Envisat) and models (NCEP,ERA-interim) After adjusting periodic signals, drifts are highlighted on Jason-1 and Envisat wind speed by cross-comparisons with models  Envisat : -10 cm.s-1 drift detected on 2003 (not seen on models and Jason-1)

26/11/2012 – Observatoire de Paris Ocean Wind speed stability over TOPEX/ERS-2 period We have also calculated global ocean wind speed differences from TOPEX and ERS-2 wind speed For TOPEX, we detect a jump (2004) and strong oscillations between 20 and 30 cm.s-1 between 1996 and 1999 and in 2001 : a strong drift is finally observed (+2.8 cm.s-1/yr) Correcting the TOPEX-B Sigma0 by delta correction not applied in M-GDR and proposed by D.W. Lockwood (NASA, 2006), oscillations are reduced but the slope remains strong.

26/11/2012 – Observatoire de Paris Ocean Wind speed stability over TOPEX/ERS-2 period We have also calculated global ocean wind speed differences from TOPEX and ERS-2 wind speed For TOPEX, we detect a jump (2004) and strong oscillations between 20 and 30 cm.s-1 between 1996 and 1999 and 2001 : a strong drift is finally observed (+2.8 cm.s-1/yr) For ERS-2, excepted a strong anomaly in 2001 (40 cm.s-1), differences are more stable and do not display patterns observed with TOPEX: a low negative drift is displayed. Correcting the TOPEX-B Sigma0 by delta correction not applied in M-GDR and proposed by D.W. Lockwood (NASA, 2006), oscillations are reduced but the slope remains strong.

26/11/2012 – Observatoire de Paris Regional MWS trends  Local wind speed trends do not highlight the same spatial structures between altimetry and models together : +/- 5 cm.s-1/yr wind speed differences TP+J1+J2 ERA-interim NCEP

26/11/2012 – Observatoire de Paris Drifts/jumps have been detected on Sigma-0 impacting the altimeter GMWS evolution : -TOPEX : dB from 1993 to 2005 with large variations between 1997 and 1999 (0.2 dB) -Jason-1 : dB from mid-2004 to Jason-2 : no drift detected -Equivalence between Sigma0 and GMWS for a mean wind speed close to 7m.s-1 is : 0.03 dB  10 cm.s-1  The Altimeter GMWS is over estimated, and the inter-annual variations between 1997 and 1999 are unrealistic  Correcting empirically these drifts on Jason-1 and TOPEX, the GMWS trend would be close to 1 cm.s-1/yr from 1993 to 2012 (instead of +2 cm/s-1/yr) : very close to the models ! Summary and conclusions Given these differences between models and considering the wind speed evolution as an indicator of the climate change, it seems important to improve the long-term stability of altimeter wind speed and therefore the sigma-0 parameter for climate studies.